In an uplink of 3GPP LTE (3rd Generation Partnership Project Long-Term Evolution, hereinafter referred to as “LTE”), a ZC (Zadoff-Chu) sequence, which is one of CAZAC (Constant Amplitude and Zero Auto-correlation Code) sequences, is adopted as a pilot signal. A frequency domain of the ZC sequence is represented by following equation 1.
                              (                      Equation            ⁢                                                  ⁢            1                    )                ⁢                                                                                                            F            u                    ⁡                      (            k            )                          =                  {                                                                                                                                                                  exp                          ⁢                                                      {                                                                                                                                                                -                                    j                                                                    ⁢                                                                                                                                          ⁢                                  2                                  ⁢                                                                                                                                          ⁢                                  π                                  ⁢                                                                                                                                          ⁢                                  u                                                                N                                                            ⁢                                                              (                                                                                                                                            k                                      ⁡                                                                              (                                                                                  k                                          +                                          1                                                                                )                                                                                                              2                                                                    +                                  pk                                                                )                                                                                      }                                                                          ,                                                  when                          ⁢                                                                                                          ⁢                          N                          ⁢                                                                                                          ⁢                          is                          ⁢                                                                                                          ⁢                          odd                                                                                                                                                                                                  exp                          ⁢                                                      {                                                                                                                                                                -                                    j                                                                    ⁢                                                                                                                                          ⁢                                  2                                  ⁢                                                                                                                                          ⁢                                  π                                  ⁢                                                                                                                                          ⁢                                  u                                                                N                                                            ⁢                                                              (                                                                                                                                            k                                      2                                                                        2                                                                    +                                  pk                                                                )                                                                                      }                                                                          ,                                                  when                          ⁢                                                                                                          ⁢                          N                          ⁢                                                                                                          ⁢                          is                          ⁢                                                                                                          ⁢                          even                                                                                                                    ⁢                                                                  ⁢                k                            =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                          [        1        ]            
In equation 1, N is a sequence length, u is a ZC sequence index of a frequency domain, and p is an arbitrary integer (p=0 in general).
In the uplink of LTE, the ZC sequence having an odd-number sequence length N is used because there are a large number of ZC sequences with low cross-correlation. In the following, a case will be explained where the ZC sequence having an odd-number sequence length N is adopted.
Also, in the uplink of LTE, a large number of ZC sequences are divided into thirty sequence groups. The sequence groups are allocated to different cells. Each sequence group includes sequences with high correlation. Accordingly, since the probability that adjacent cells use the sequences with high correlation is reduced, the interference between the adjacent cells is reduced. In this case, the sequences with high correlation mean sequences having a similar u/N (u: sequence index and N: sequence length). When inter-sequence interference is low, it is difficult to demultiplex the sequences having a similar u/N.
In each sequence group, an allocation bandwidth (in particular, the number of allocation resource blocks (RBs)) is associated with one or two sequences. To be more specific, one sequence is associated with each number of RB for the allocation bandwidth of 5 RBs or less, and two sequences are associated with each number of RB for the allocation bandwidth of 6 RBs or more.
FIG. 1 illustrates the sequence groups adopted in the uplink of LTE. Focusing on each sequence group as illustrated in FIG. 1, for the allocation bandwidth of 5 RBs or less, only a sequence index u corresponding to a u/N that is closest to a u/N (for example, u=1 and N=31 in sequence group 1) obtained when the number of RB is 3 (hereinafter, this sequence is referred to as “#A”) is allocated to the number of RB. Meanwhile, for 6 RBs or more, a sequence index u corresponding to u/N that is second closest (hereinafter, this sequence is referred to as “#B”) is also allocated to the number of RB besides the above #A. Although one sequence is allocated to each of 1 RB and 2 RBs, the allocated sequences are different from the ZC sequences and therefore the description thereof will be omitted.
Inter-slot hopping is applied to a pilot signal in the uplink of LTE (see, for example, non-patent literature 2). A sequence group including the sequence used as the pilot signal discreetly changes among slots (that is, performs hopping). This inter-slot hopping pattern is cell specific. Thus, the inter-slot hopping randomizes inter-cell interference.
To be more specific, the patterns of the inter-slot hopping (hereinafter, referred to as “inter-slot hopping pattern.”) include pairs of seventeen hopping patterns and thirty shifting patterns. The same hopping pattern is applied in each cell in one cell cluster (in other words, cells forming one group), but different hopping patterns are applied in cells in different cell clusters. In the one cell cluster, a different shifting pattern is applied in each cell. In particular, in FIG. 2, the same hopping pattern (+2, +3, +3, . . . ) is applied in cell#1 and cell#2 that belong to the one cell cluster. Meanwhile, as the first sequence group, SG#1 is set in cell#1 while SG#2 is set in cell#2. This means that the shifting patterns applied in the cells belonging to the one cell cluster are different. Specifically, the shifting pattern can be considered as an initial value of the inter-slot hopping pattern.
The inter-slot hopping described above can prevent the same sequence group from being used in the same slot in a single cell cluster. Meanwhile, among different cell clusters, the same sequence group may be used in the same slot. However, since the cell clusters use different hopping patterns in this case, it is possible to prevent the same sequence group from being continuously used in a plurality of slots between any cells. Also, a base station can select whether or not to apply (enable or disable) the inter-slot hopping. The base station notifies terminals of this selection result through higher layer signaling. In case of applying the inter-slot hopping, there is no need to change the above sequences #A and #B for each slot and therefore only sequence #A is used. This is because the sequence groups are different between predetermined slots according to the inter-slot hopping, and therefore the sequences applied to the following slots are also different.
In case of applying the above inter-slot hopping, different sequence groups are used between the slots. Thus, focusing on a certain cell, interference to a pilot signal transmitted in the cell from other pilot signals (that is, inter-sequence interference) differs in each slot.
Furthermore, LTE-Advanced (hereinafter, referred to as “LTE-A”) aims at further reduction of the inter-sequence interference of the pilot signal in uplink. Using OCC (orthogonal cover code), which is an orthogonal sequence, in combination with the above ZC sequence to reduce the inter-sequence interference is under study. The multiplexing scheme adopting this OCC multiplies pilot signals mapped to continuous two slots by w1=[1 1] or w2=[1 −1] (see FIG. 3). When multiplied by w1, the same pilot signals as the conventional pilot signals are mapped in the first and second slots. Meanwhile, when multiplied by w2, the same pilot signal as the conventional one is mapped in the first slot and a pilot signal whose phase is inverted (that is, rotated by 180 degrees) is mapped in the second slot. In LTE, the scheduling is performed on a subframe basis. The subframe is composed of two slots.
A reception side of the pilot signal performs channel estimation by multiplying the symbols to which the pilot signals are mapped in two slots by a complex conjugate of the ZC sequence, and multiplies the acquired channel estimation value by a complex conjugate of OCC for combination. By this means, a channel estimation value of the desired component is combined in-phase and a channel estimation value of the interference component is combined out-of-phase and removed. When the same ZC sequences is used in two slots, the channel estimation may be performed by multiplying a symbol to which a pilot signal is mapped in each slot by the complex conjugate of OCC for combination and then multiplying the result by the complex conjugate of the ZC sequence.
However, when inter-sequence interference (interference component) among the pilot signals is different in each slot due to the application of the above inter-slot hopping, the interference component is not combined out-of-phase and therefore the interference component cannot be removed. For example, even when UE#0 transmits a pilot signal multiplied by OCC sequence w1 and when UE#1 transmits a pilot signal multiplied by OCC sequence w2, a large number of interference components remain in each received pilot signal subjected to demultiplexing processing at the reception side. Also, since each cell uses the same sequence group without the application of the above described inter-slot hopping, the inter-cell interference cannot be randomized and the inter-sequence interference obviously occurs.
To solve this kind of problem, non-patent literature 3 proposes addition of inter-subframe hopping in LTE-Advanced. As the inter-slot hopping, this inter-subframe hopping is also set by the higher layer signaling or PDCCH. To be more specific, the inter-slot hopping performs hopping of the sequence group among slots and the inter-subframe hopping performs hopping of the sequence group among subframes. That is, the inter-slot hopping is identical to the inter-subframe hopping but differs in whether a hopping unit is based on a slot or a subframe. According to the inter-subframe hopping, the inter-cell interference of the pilot signals can be randomized for a terminal to which a data transmitting resource is allocated over a plurality of subframes (for example, a retransmission terminal and a persistent terminal). Also, according to the inter-subframe hopping, since the slots use the same sequence group, the OCC can orthogonalize the pilot signals mapped to consecutive slots.